The present invention relates to carbonate-containing mineral fillers, pigments and similar materials.
Fillers are generally understood to be relatively inexpensive substances which are mixed for example with materials, adhesive and coating materials, paper, plastics and the like in order to increase the volume and/or the weight of them, but often also to improve the industrial usability of them. It is also possible, by means of suitable additives to improve the quality e.g. the hardness, strength, elasticity and expansion of rubber and synthetic elastomers (the additives here being known as accelerators).
Pigments are inorganic or organic, multi-colored or monochromatic coloring agents which are practically insoluble in the medium in which they are used. Many inorganic pigments also act as fillers, and vice versa. Pigments are used in particular for coloring lacquers and paints, the coloring of plastics, paper, textiles, cement, concrete, ceramics, glass and enamel, cosmetics and foodstuffs, as well as for color printing in the graphics industry and as artists colors.
In the paper, paint and lacquer industries in particular products are required which have a high gloss and a high opacity. German Pat. No. 27 33 722 relates to a method for the continuous production of calcium sulphoaluminate pigment with a particle size of from 0.1 to 2.0 .mu.m for paper coating by reaction of calcium hydroxide with aluminium sulphate. This method gives, among other things, a high gloss in printing papers, even under moderate calandering conditions on account of the high degree of hydration of gloss white, paper with a high opacity being obtained by the formation of a thick coating layer. German Pat. No. 1,938,162 relates to a method for coating paper or the like, in which a coating material consisting of a polymer composition contained in a liquid medium is applied to a web. This method is claimed to make possible the direct production of a light coating of high smoothness and opacity without the necessity of subsequent glazing.
German Auslegeschrift No. 20 26 963 relates to a method for producing a paper based on natural and/or synthetic fibres or a foil made of a synthetic material, and in particular a reproduction paper. In this method a methylene urea precipitate slightly polymerized in a strongly acid medium is incorporated into the fibre pulp as a white pigment or applied as a coating to the surface. In this way it is claimed that an improvement in the whiteness and an improvement in the opacity of the paper is achieved.
A further important property associated with papers is the retention value. The retention value indicates the amount of filler mixed, for example, in the pulp remains in the finished product. This will be explained in greater detail by the example of paper-making.
Fillers are understood to be materials which are used added to the paper.
It has long been known in the paper-making industry that costly fibres which are also relatively difficult to obtain can be replaced by fillers of mineral origin. The filler content of the various types of paper is between 10 and 30%. A further advantage of the use of fillers in paper production lies, among other things, in the fact that the finely distributed filler substances fill the interstices between the individual fibres and through this make the surface of the paper smoother and glossier and reduce the transparency of the paper, which is a fundamental requirement in the case of printing and writing paper types. A disadvantageous phenomenon, however, lies in the fact that by increasing the quantity of filler the size content of the paper is reduced or more resin is required to obtain the same size content, and also that the tearing length of the paper is reduced.
Fillers are usually classified, on the basis of their chemical composition, into silicates, sulphates, carbonates and oxides. Kaolin, which belongs to the silicate group, is the most widely used in the paper industry. The main reason for the wide use of kaolin is the relatively inexpensive price of it and in its retention of about 60-70%. The retention value indicates the quantity of filler mixed in the pulp that remains in the finished product.
There are limitations to the use of chalk, which belongs to the carbonate group, despite its technologically favourable properties, imposed by its retention of about 50-55% as compared with kaolin. The chalk used as a filler is produced from the residues of supporting structures of microorganisms, which occur frequently in nature and are for the most part of microscopic fineness, the so-called nanofossils, by suspension. By precipitation from calcium chloride solution by means of sodium carbonate or from calcium hydroxide solution by means of carbon dioxide, so-called precipitated calcium carbonate is obtained.
Apart from its fairly poor retention, natural or precipitated calcium carbonate has as a further technological disadvantage the property that it is very sensitive to the aluminium phosphate sulphate used to precipitate the sizes.
German Pat. No. 27 37 742 is based on the problem of using by-products in the paper industry as fillers. The solution of this problem lies in the use of the calcium carbonate occurring on the causticizing of sulphate waste lyes from cellulose production as a filler for paper production, the calcium carbonate being used as a maximally 75% aqueous suspension in the form of the resultant lime slurry, after the standardization of its pH of between 5 and 12 by an acid or mixture of acids.
Further documents relating to retention are German Pat. No. 25 51 259 and German Auslegeschrift No. 1,546,240
Retention agents are also referred to as flocculation agents.
Fibrous subtances and fillers are mixed together in the pulp in the hollander or vat. The object is to bring both subtances to the screen of the paper-making machine as an homogeneous pulp, as far as possible without losses due to sinking in the liquor or washing out, in the given quantitative ratios. Here, however, fillers and fibrous substances behave quite differently as regards dispersibility, so that fillers may still occur in the effluent. A measure of this behaviour is the "total retention", the percentage ratio of the quantity of filler in the finished paper to the quantity of filler (absolutely dry) which was added altogether (including the fillers contained in the return water=white water). As high as possible retention (retention capacity) is aimed at, a large number of factors being involved in the physically complicated colloidal structure of the system fibre/filler/water, but not all being able to be varied under practical conditions.
A further important property in association with the use of pigments and similar materials in dyes and lacquers, papers and plastics is abrasiveness.
According to DIN 50320, the term "wear" is understood to mean "an undesirable change in the surface of objects in use through the loosening and separation of small particles through mechanical causes". In German usage the word "abrasion" borrowed from English is approximately equivalent to the concept of wear. Unlike DIN 50320, however, the term "wear" in the specialized literature covers the action of mechanical, chemical and thermal components (cf. the term "corrosion").
In the paper industry fillers occur suspended in water together with the fibres and adjuvants. They come into contact with rotating and stationary particles or pass between machine component surfaces which are in motion in relation to each other, e.g. the suction box and the screen of the paper-making machine. In these cases they cause damage to the surfaces of the machine components. Suspended pigments as components of coatings behave in a similar manner.
To determine numerical data relating to the abrasive action of fillers in aqueous suspension laboratory tests have been developed, for example by means of the Einlehn AT 1000 Abrasion Tester.
In North America and Scandinavia the Valley Tester is used with testing procedures specific to the apparatus is used for the same purpose.
Since the individual phenomena determining abrasion depend on the process and the equipment, the measurement method used must also be indicated together with the measurement results (A. Breunig and W. F. Hill, Verein Zellcheming, Berliner Allee 56,D-6100 Darmstadt, pamphlet V/27.5/75, published Oct. 23, 1975).
A further important requirement is as good an adhesion as possible between the particles and the coating agent in order to obtain better mechanical properties in the end product (David L. Skinner and Edward L. Moon, 27th Annual Technical Conference, 1972, Section 15-F, pages 1 to 4).
As is known, particularly fine-grained mineral fillers are now produced according to tne state of the art by wet grinding and subsequent drying. The products obtained are used in particular in the paint and lacquer, plastic and paper industries. Organic dispersion agents such as polyacrylates are normally used during the wet grinding. Products manufactured in this may contain very many agglomerates after drying and are characterized by poor dispersibility.
One of the most important properties of fine-grained fillers, pigments and similar materials, however, is their dispersibility, for example in plastics such as PVC, polycarbonate, polystyrene and polyethylene, as well as in lacquer artificial resin binders, softeners, papers, etc. In these cases as fine as possible a distribution of the filler or pigment in another material, for example in the aforementioned plastics, lacquers, papers, etc. should be achieved. The methods suitable for dispersion are grinding methods, or also apparatus based on the principals of ultrasonics. Generally speaking, dispension media are used in addition. Dispersion media are organic or inorganic, monomer or polymer substances which facilitate the dispersion of particles in a dispersion agent by reducing the surface tension between the two components, i.e. by wetting.
Dispersion media are substances with the property of preventing agglomerations or aggregations of the particles of pigments, fillers or resins and bringing these into a fine distribution or also countering in advance a tendency to agglomeration or even precipitation. They have surface-active properties and are used in the preparation (grinding) of fillers and dye pigments for coating compositions; and for a better distribution of resin-free pulps. Among those that can be mentioned are sodium hexametaphosphate, sodium pyrophosphate, alkylphenol-polyglycol ether, and alkyl-aryl sulphonic acid salts. The use of them should be confined to the smallest concentrations.
Readily dispersible inorganic pigments, the surfaces of which bear non-drying, fatty-acid-modified alkyd resins based on polycarboxylic acids, polyols and fatty acids with more than 6 C atoms, are already known from German Auslegeschrift No. 20 01 381.
German Offenlegungsschrift No. 24 56 463 discloses a method of preparing fine-grained calcium carbonate dispersions by homogenization of precipitated calcium carbonate in the presence of a dispersion medium into a suspension containing 15-25% water and subsequent wet grinding of the suspension, in which, for the purpose of producing readily dispersible calcium carbonate of a high degree of fineness and density, the dispersion is transformed into a free-flowing product.
German Pat. No. 29 08 699 describes a method of producing powder pigments with improved dispersion properties, in which the pigments are passed by known methods into a drier, the surface-active agent is introduced directly into the charging zone of this drier, which is separate from the actual drying zone, and on to the aqueous pigment pulp, after which the coated pigment is dried in the drying zone of the drier.
A method is disclosed in German Auslegeschrift No. 29 21 238 for the preparation of readily dispersible pigment preparations with a pigment content of more than 65% by weight, in which the pigment is suspended in water and in which a salt of an organic acid is used. This method is characterized by the fact that an octoate is used as the organic acid salt.
Finally, a method is known from German Pat. No. 23 46269 for the preparation of highly concentrated fine-grained calcium carbonate dispersions in which the calcium carbonate obtained by reaction of an aqueous calcium salt solution with a CaO content of 7-14% by weight with an aqueous alkali carbonate solution which contains 150-250 g/l CO.sub.2 as carbonate and 5-10 g/l CO.sub.2 as bicarbonate is first homogenized into a suspension containing 15 to 25% by weight water and this, after possibly further dilution to at the most 40% by weight water, is subjected to wet grinding. It is claimed that by this method highly concentrated, fine-grained calcium carbonate dispersions with CaCO.sub.3 grain sizes of less than 5 .mu.m and which contain up to 80% by weight calcium carbonate can be prepared.
Fine-grained calcium carbonates are nowadays generally prepared by wet grinding, and for use in the paint and lacquer industry, the plastic industry and in some cases even for use in the paper industry, are dried. During this drying agglomerates are formed which distinctly reduce the dispersibility. Consequently an obvious measure is to change to a dry grinding in which the disadvantage of agglomerate formation due to the drying process does not exist. Dry grinding methods of this kind are already known (e.g. from U.S. Pat. No. 3,022,185).
These methods, as can be expected, actually also lead to products which have a better dispersibility than those that have been obtained by wet grinding and subsequent drying. Here, of course, it must be borne in mind that in the dry known methods of dry grinding products with a coarser grain-size distribution than in the wet-grinding methods of the prior art are always obtained, which, on account of the larger particles, also automatically have a poorer dispersibility, so that, viewed as a whole, the dispersibility of the products that are obtained by the dry method is only insignificantly better than the dispersibility of the particles that are produced by wet grinding and subsequent drying.